This invention teaches a method and means for scanning an object field to produce an image, wherein the aim is to achieve as great a performance as possible in a compact equipment. In order to achieve this aim it is necessary to ensure that the total optical system can be corrected to the limit set by diffraction; that the smallest possible detectors available can be utilized; that an array of many detectors can be used, if necessary, to increase total performance, without sacrificing the individual performance of each detector; that a hgh relative aperture of the whole optical system can be achieved if required; and that a wide angular field of view can be scanned in two directions.
A further aim of the invention is to utilize methods of achieving the above technical aim in a manner which will minimize the cost of fabrication.
The scanning means described in this invention has many features in common with Harris and Neiswander (U.S. Pat. No. 3,508,068) or Maxwell (U.S. Pat. No. 3,588,517) who teach a means of scanning wherein a fixed primary optical objective focuses a radiation image on to a spherical focal surface, in which focal surface is mounted a series of apertures on a rigid member. With Harris and Neiswander the apertured rigid member is fixed in position, the apertures being scanned by an optical relay system rotating about an axis passing through the center of curvature of the spherical image surface, whereas with Maxwell the apertured member rotates about an axis passing through the center of curvature of the spherical image surface, and with each aperture is associated a small relay lens fixed to and rotating with the said apertured member.
In the invention of Harris and Neiswander radiation is directed off the rotating member, to be focused on the axis of rotation of the rotating member by means of a multi-sided pyramidal reflecting prism. In the case of Maxwell the radiation is focused at the center of curvature of the focal surface. In either case a stationary detector is placed at the final focal point of the radiation whereby to provide an electrical signal which is amplified and presented on an output device to form an image.
In the invention of Harris and Neiswander the basic invention describes only a single line scan in (say) the horizontal direction. To provide a vertical scan a second device such as a rotating plane mirror is interposed between the scanning device and the object field; or the whole scanning system is mounted on a platform which is rotated about a horizontal axis. Maxwell's invention is specifically devised with its arrangement of apertures to provide a two-dimensional scan by use of a single detector, and without a rotating plane mirror.
If an array of detectors were to be used for greater performance with respect to a single detector, difficulty would be experienced in the Harris and Neiswander arrangement due to rotation of the image of the detector array projected through the moving optical system, relative to the apertures in the grid rotating in the primary focal surface. This difficulty would not be experienced with the arrangment of Maxwell wherein the radiation is focused at the center of curvature of the focal surface.
The angular resolution of the scanning systems of the inventions of Harris and Neiswander and of Maxwell are determined by the dimension of the scanning apertures relative to the focal length of the primary optical system. In order to resolve the angular resolution so calculated, simultaneously with utilizing the full available aperture of the optical system, it is essential to use a highly corrected primary optical system. In both Harris and Neiswander and Maxwell, a concentric correcting element of the Bouwers-Maksutov type is described in conjunction with a spherical mirror. Where infrared radiation is to be passed by the scanning system, the transparent material required for the correcting element would be expensive.
To achieve compactness of the scanning arrangement, which is part of the aim of this invention, requires both the focal length and the entrance aperture of the primary optics to be as small as possible consistent with a required performance. To achieve high resolution, simultaneously, demands such small apertures in the scanning grids referred to above as to render them difficult to fabricate. For example, a resolution of 0.1 milliradian with optics of 75 mm focal length and 100 mm optical aperture, requires a grid aperture size of 0.0075 mm.
Furthermore, the Bouwers-Maksutov correcting element in front of the spherical primary mirror must be made of material transmitting to the incident radiation and must be fabricated very precisely by grinding and polishing to very close tolerances. If a very wide field of view, for example 150.degree., is required, the resulting dome is very deep which is difficult to work, and if made of infrared transmitting material requires a substantial amount of expensive material. The expense of the material and the cost of working the correcting component are avoided in this invention, because the function of this large component is performed by a number of very much smaller components which may be made of cheaper materials.
The arrangement of Stephenson (U.S. Pat. No. 3,899,145) also has features in common with the arrangements of Harris and Neiswander, and Maxwell, in using a Bouwers-Maksutov correcting element in front of the primary spherical mirror to correct aberrations. However Stephenson's arrangement is designed to project a laser beam rather than to collect radiation at low intensity and focus the radiation on to a small detector. Specifically, the arrangement does not permit multiplication of the sets of optical elements about the axis of rotation of the rotating member to provide several scans for each revolution of the rotating member; and the detector used has, relatively, a very large area.
As described previously the arrangement of the present invention dispenses with the need for a correcting element in front of the primary mirror; enables the radiation to be focused precisely on to a very small detector approaching in size the diffraction limit of the optical system; permits the use of a large number of detectors in an array to achieve greatly enhanced performance over a single detector; and permits multiplication of the sets of optical elements on the rotating member about the axis of rotation of that member.
In an arrangement described by Bez (U.S. Pat. No. 3,864,567) a multiplicity of primary optical systems is mounted on the rotating member with other components of the optical systems. The result is an arrangement which would require much greater driving power, and a heavier construction, for a given performance, than would be required in the arrangements of Harris and Neiswander, Maxwell, Stephenson, or of this invention.
Alternatively, Aulin (U.S. Pat. No. 3,206,608) describes an arrangement for scanning an image which has some features in common with this invention and those of Harris and Neiswander, Maxwell and Stephenson, but dispenses with the correcting element in front of the primary collecting mirror. In Aulin's arrangement a negative lens element is shown which is disposed to focus the radiation from the primary mirror on to the center of curvature of the primary mirror, and which is mounted on a rotating arm whose axis of rotation passes thru the center of curvature of the primary mirror, a detector being mounted at this center. It can be shown easily that a high relative aperture and a definition approaching the diffraction limit could not be achieved simultaneously by this simple arrangement, but can be achieved by the arrangement of this invention.
In addition to the above-described improvements on the prior art, this invention includes the following features to improve the overall performance:
(1) introduction of an oblique mirror in front of the primary mirror and the rotating member, to reduce the obstruction of the incoming radiation by the rotating member PA1 (2) means to minimize noise in the detector by eliminating unwanted radiation from the detector PA1 (3) a number of means for introducing a second scan PA1 (4) means for stabilizing the line-of-sight in a compact arrangement PA1 (5) means for removing rotation of the axis of the cone of rays incident on the detector PA1 (6) means for scanning a plane or spherical object field at a finite distance in front of the primary spherical mirror. PA1 (a) in common with Aulin dispenses with the need for a correcting element in front of the primary mirror, as required by Harris and Neiswander, Maxwell, and Stephenson, but permits the achievement of a much higher relative aperture than with Aulin's arrangement, whilst achieving simultaneously a definition closely approaching the diffraction limit of the optics, PA1 (b) eliminates the need for an element containing a multiplicity of apertures to determine the definition of the system, as with Harris and Neiswander, and Maxwell, PA1 (c) by permitting the use of several detectors simultaneously, enables very much greater performance to be achieved than with a single detector, PA1 (d) by introducing an oblique plane mirror in front of the primary mirror, reduces the obstruction of the incoming radiation by the rotating assembly of scanning optical elements, PA1 (e) introduces means for minimizing the noise in the system due to unwanted radiation entering the detector, PA1 (f) permits the scanning of a plane or curved object field at a finite distance in front of the primary mirror, PA1 (g) permits a two-dimensional field to be scanned in a number of ways; in one example, the second scan being 360.degree., PA1 (h) permits stabilization of the line-of-sight, PA1 (i) removes rotation of the axis of the cone of rays incident on the detector.
The means for achieving these optical features will become clear in the following description of the preferred embodiments.